Horizontal landfill gas wells: Geometry, physics of flow and connection with the atmosphere

Abstract

Landfills form an important link in the chain of waste management. Sound engineering design and effective gas collection can only be attained via a thorough understanding of flow in different parts of the landfill. This study addresses gas flow induced by perforated horizontal wells commonly used for landfill gas collection. Planar flow through a porous medium of varying properties is solved numerically, yielding a twofold insight: quantitative deviations establish the validity and limitations of theoretical models, whilst qualitative comparison illustrates the impact of various flow parameters. It is found that an accurate modelling of geometry and medium resistance is essential in determination of a correct pressure distribution that can then be used to optimise gas collection. Gravity, an often overlooked effect, is shown to be non-negligible. The error source and bounds are given and explained for different landfill configurations. By contrast, temperature is of a lesser importance by an order of magnitude, implying that comprehensive spatial modelling of the exothermal reactions generating the gas as well as temperature dependent fluid properties such as viscosity are of a secondary interest. The well radius of influence, i.e. the maximal collection distance, is a fundamental design parameter impacting the interchange of fluid with the atmosphere. The numerical solution permits to obtain optimal values for effective collection and minimise flux between the landfill and atmosphere. Based upon a comprehensive numerical simulation an analytical approximation is suggested as an accessible design tool. The formula enables the computation of pressure profiles within the landfill and involves a minimal number of required input parameters. The surface flux and radius of influence can also be determined thereby.